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Key Points

IBD is comprised of two inter-related disorders, Crohn disease (CD), and ulcerative colitis (UC), as well as indeterminate colitis (IC). IC describes the 5% to 10% of IBD involving only the colon and is neither classic CD nor UC.

Anatomic involvement: CD can involve any portion of the gastrointestinal (GI) tract, most commonly the terminal ileum (5%-75%) and colon (20%-50%). CD causes transmural inflammation in all four layers of bowel wall; extension through the bowel wall can cause fistulae, abscesses, or perianal complications. Strictures secondary to fibrosis are relatively common. UC is localized to varying lengths of continuous inflammation proceeding proximally from the rectum; a discontinuous cecal patch may be present. UC is limited to mucosal and submucosal inflammation, and does not cause perianal or intra-abdominal disease; strictures are much less common than in CD.

No single gene cause of IBD is known to exist except perhaps for a rare form of CD (see IL-10 pathway later).

Family history:

An affected first-degree relative confers a relative risk of 30 to 40 for CD and 10 to 20 for UC. Overlapping risk occurs since first-degree relatives of patients with CD have relative risk of 3.9 for developing UC.

Twin studies:

Monozygotic twins have a 20% to 50% concordance rate in CD, much lower in UC.

Environmental factors:

Luminal microbes or microbial products are implicated as “environmental” triggers for IBD.

Colonoscopy with biopsy and ileum cannulation. Granulomas are present on endoscopic biopsies in approximately 10% of CD, crypt abscesses are indicative of UC.

Serologic markers may be helpful; anti-Saccharomyces cerevisiae antibodies (ASCA) and antineutrophil cytoplasmic antibodies with perinuclear staining (p-ANCA) have high sensitivity for IBD in combination compared to other causes of intestinal inflammation, but sensitivity varies for different ethnic groups (much lower, eg, in regions endemic for tuberculosis). ASCA shows high specificity for CD, and p-ANCA for UC, although patients with CD limited to the colon may be positive ...